The Problem
Methods for preparing membranes containing pores of dimensions useful for filtering out substances of various sizes, including salts in solution and viruses, are known. However, none of these disclosed methods has been found to be satisfactory for preparing membranes of vinylidene fluoride polymer having a pore size range suitable for ultrafiltration. Furthermore, known solvent casting methods of forming porous vinylidene fluoride polymer sheets require that the sheets be held under restraint during the forming process to avoid uneven shrinkage causing a wrinkled, rough-surfaced product.